In a number of industries, and in particular the pharmaceutical industry, processes are used that generate fume streams that are either batched or highly cyclical. Often, these fume streams may vary with time in either flow rate, composition, concentration, or in any combination thereof. A depiction of the fume flow rate profile versus time may show a relatively short time duration of very high or rich flow, a period where the fume flow rate rapidly drops off, and a long period of relatively low flow rate. This flow rate profile may then repeat itself indefinitely.
Regional, state, or federal air pollution regulations often require that such fumes be abated, but dealing with the cyclical, highly variable fume profiles poses a particularly difficult problem when designing an abatement system. A conventional thermal oxidation system, designed for the low flow or "nominal" condition, will be overwhelmed during a high flow condition. During periods of high flow, the system will perform poorly, for example achieving poor destruction and removal efficiency (DRE), while leaving a high CO content and high formation of products of incomplete combustion (PIC's ). Even though the duration of these high flow periods is short, the large quantities of unoxidized VOC's or PIC's can completely offset the long periods of good system performance to produce a system with overall poor performance..
On the other hand, if a conventional thermal oxidation system is designed for a high flow condition, then the oxidizer will operate with acceptable environmental performance at all times. The problem with designing for the high flow condition is that, during the long periods of low flow, large quantities of supplemental fuel will-have to be burned to keep the system on line due to the limited turndown of the thermal oxidizer. This high fuel consumption will also lead to detrimental NO.sub.x generation.
Often, if the system is designed for the high flow condition, the thermal oxidizer may require additional oversizing to prevent flameout during those periods of dramatic flow fluctuation when fume flow either dramatically increases, or dramatically decreases. This oversizing may be such as to allow three to ten times the flow that is required to oxidize even the maximum flow case. Such design choices dramatically affect the overall system size, cost, and thermal efficiency.
These problems, or other similar problems, apply to any of the conventional abatement alternatives currently available, including incineration, thermal oxidation, regenerative thermal oxidation, recuperative thermal oxidation, and catalytic oxidation.
Another abatement alternative is to use a porous inert matrix thermal oxidizer such as is described in U.S. Pat. No. 5,165,884 (Martin et al.). This is much better than the conventional abatement alternatives because the processor need be sized for no larger than the maximum flow condition and the thermal mass of the matrix can actually be utilized, resulting in a unit that can be sized for smaller than the maximum case. In such a design, during over-design-flow cases, the process operates in an off-equilibrium condition, but the thermal mass of the matrix dampens the effect of the flow fluctuations until the fluctuation passes and the control system brings the processor back into equilibrium.
Even the porous inert matrix thermal oxidizer system has some limitations. Sometimes the high fume flow case is so much larger than the nominal flow that excessive system pressure drops may be produced (in adherence to the Bernoulli equation, which states that pressure drop varies proportionately with velocity squared).
At other times, the high fume flow case is of such duration that a small sized processor would experience "thermal breakthrough" before the flow drops back to the nominal flow and allows reestablishment of the equilibrium condition. Thermal breakthrough occurs when the flow is high enough for a long enough time period that the reaction wave migrates out of the matrix entirely, leaving behind it only cold matrix. The simple solution to this problem is to install a processor that is larger, but still not as large as a processor sized for the maximum flow condition. However, this generates additional costs since the processor size increase will also necessitate a preheater system size (and cost) increase, as well as other modifications of the overall system.
This can be especially problematic when a preheating system is preferred that may possess certain inherent performance advantages, but which may possess an escalated level of cost. For example, an electrical supplemental heating system may be desirable for its ease of utility installation, excellent environmental performance at all times, and heightened safety features. However, the electric preheat package is typically more costly than an all-gas preheat package, and electricity consumption is typically three times as costly as natural gas consumption. Therefore, a relatively small increase in processor size could cause an inordinate increase in preheat capital and operating cost for such a system.
It can be seen that there is a need for a method and apparatus that will achieve adequate DRE's of VOC-containing process fumes that is capable of efficiently handling large variations in fume flow rates. In particular, there is a need for a system that can handle batch processes and processes having cyclical heat content or fume flow profiles.